This invention relates generally to transceivers used in mobile communications, and more specifically to universal receivers capable of receiving both analog and digital FM signals.
The new trend in mobile communications is towards integrated voice and data services. Voice is the most commonly used mode of communications for network subscribers; whereas, data transmission is required for sending control commands between a mobile terminal and the network management center. Recently, alphanumeric message transmission has also gain popularity with the network subscribers as a more cost effective means of communications. For system simplicity as well as economy, it is desired to employ a single transceiver to accommodate both analog (voice) and digital (data) signals. Conventionally, voice services have utilized analog frequency modulation (FM), while data services have had a tendency to employ frequently shift keying (FSK) or differential phase shift keying (DPSK) techniques.
In the integration of two modulation modes of analog (voice) and digital (data) transmission, no performance degradation should be allowed. In other words, the fact that a particular design is dual purpose should have no impact on the performance of either of the two modes of operation. Desirably, such a unit will yield good power performance for both analog and digital signals.
A paper by F. Davarian, M. Simon and J. Sumida, "DMSK: A Practical 2400-bps Receiver for the Mobile Satellite Service, JPL publication No. 85-51, June 1985, describes a robust receiver for digital signals in a mobile environment. This receiver uses differential minimum shaft keying (DMSK) which is a form of frequency shift keying. A conventional DMSK receiver demodulates a frequency modulated signal by a KT delay and multiply operation, where K is an integer and T is the bit time. The generalized block diagram of the receiver when delay D equals T is shown in FIG. 1.
A paper by K. Fonseka and N. Ekanayake, "Differential Detection of Narrow-Band Binary FM," IEEE Trans. Commun., July 1985, considers differential detection of narrowband binary FM, using a detector which employs a delay line whose delay period is dependent on the value of the modulation index of an associated transmitter. The error-rate performance was computed for the receiver and compared with that of a comparable limiter-discriminator (L-D) receiver. It was shown by the comparison that differential detection yields performance comparable to L-D detection.